The present invention relates to a display drive circuit used for driving various display devices such as a fluorescent character display tube, a plasma display, a LCD display and a LED display.
FIG. 8 shows a conventional display device which displays an arbitrary pattern such as characters using a plurality of display elements such as LEDs. This display device has a drive circuit 4 for selectively providing a drive current to one LED 2 of a LED module. In the drive circuit 4, transistors 8, 10 are provided on the cathode side of the LED 2 and form a switching circuit that intermittently turns on the LED 2 in response to a common signal COM, and a transistor 12 is provided on the anode side of the LED 2 and turned on in response to a color signal Ci. That is, a series circuit is formed by a resistor 14 and a transistor 12 that are provided on the side of a power supply line 16, and a transistor 10 provided on the ground side. The color signal Ci decreases the base voltage of the transistor 12 to a low (L) level to turn on the transistor 12. During a L-level period of the common signal COM, the transistors 8, 10 are turned on, so that a drive current intermittently flows through the LED 2 to turn it on as long as the color signal Ci is provided to the LED 2. Although the LED 2 is turned on intermittently in accordance with the common signal COM, the human eyes recognize the lighting state of the LED 2 as continuous because of high-frequency switching by the common signal COM.
However, the above display device can drive only one LED 2 (one color) per one common signal COM. In order to perform a color display using two or more LEDs (two or more colors), the drive circuit 4 should be scaled up in accordance with the number of colors. Thus, the number of components, such as transistors, of the drive circuit 4 will increase proportionally, and the circuit configuration becomes complex.
As shown in FIG. 9, assume here that an LED module 2M, in which LEDs 2 are arranged in a matrix form, is driven by dynamic scanning. If a clock signal, that is a basis of the common signal COM, is stopped, the transistors 8, 10 of the drive circuit 4 (see FIG. 8) are turned on continuously and all of a plurality of LEDs 2 constituting one line 2L are turned on. An overcurrent flowing through each of the LEDs 2 constituting the line 2L will reduce the life of the LEDs 2 of the line 2L. In the worst case, the LEDs 2 will break down.